The present invention relates to a rectangular battery comprising an electrode group composed of positive electrodes and negative electrodes, inserted and airtightly sealed in a rectangular battery case.
With the recent increase of small size portable apparatuses the demand for rechargeable batteries has also increased. And especially with the development of small size, thin type and space efficient apparatuses, the demand for rectangular rechargeable batteries has rapidly augmented.
The rectangular rechargeable batteries are composed of an electrode group placed into an airtightly sealed rectangular battery case. In the widely used alkaline rechargeable batteries, the positive electrodes use a nickel hydroxide as the active substance and a hydrogen absorbing alloy is used in the negative electrode. Collector tabs are spot welded and electrically connected to the positive electrodes and to the negative electrodes which are stacked with separators as insulators. The electrode group is placed in the rectangular battery case and airtightly sealed. The collector tabs, which are connected to the positive electrodes, are connected to the positive pole terminal.
The metallic battery case that includes the electrode group generally has its open portion airtightly sealed by means of a laser welded or caulked metallic cover. The method used to airtightly seal the open part of the rectangular battery by laser welding does not require an open part with local very special properties, compared with the open part of a battery case to be sealed by caulking.
The rectangular batteries mounted in electronic apparatuses have excellent space efficiency properties. But compared with cylindrical type batteries, they have the drawback of a worse volumetric energy density. For example, the volumetric energy density of a nickel hydrogen cylindrical battery reaches 200-220 Wh/1, and for a nickel hydrogen rectangular battery it is reduced as much as 170-190 Wh/1. Because rectangular type batteries have excellent space efficiency properties, if the volumetric energy density could be improved, in the condition where the batteries are grouped, the volumetric energy density can be fairly augmented.
To allow the improvement of the volumetric energy density of the rectangular type batteries, the electrode group composed of stacked positive electrodes and negative electrodes, has to be strongly pressed to high density and inserted into the battery case. However, in comparison with cylindrical batteries, it is extremely difficult to introduce into the battery case the high-density positive and negative electrodes of the rectangular batteries. This is because when inserting the electrode group, one part of the active substance is peeled off at the open part of the battery case and falls off from the surface of the core. The active substance that has fallen off from the core, not only degrades the electrical characteristics of the electrode group, but also considerably reduces the efficiency of the rectangular battery. The reason is that the active substance that has adhered to the open part of the battery case impedes the airtight welding of the open part. When airtightly welding the circumference of the cover to the open part of the battery case of a rectangular battery by laser welding, the airtight sealing of the cover on the battery case will become impossible if some active material, in even a very small amount, adheres to the region to be laser welded. In the case of the structure where the battery case is airtightly sealed by caulking, it is extremely difficult to airtightly seal the corners and furthermore, because the active substance tends to easily adhere to these parts, the airtight sealing of the battery case is extremely difficult. For this reason, even if there is a demand for rectangular batteries with an electrode group having a much higher density inserted under pressure into the battery case, it becomes more difficult to insert inside the battery case under pressure and with a high density, the electrode group from which the active material has fallen off of the core. Therefore, it is fairly difficult to improve the volumetric energy density of a rectangular battery.
Rectangular batteries have two problems, as is hereunder described, that are not applicable in cylindrical batteries, and which make the insertion of a high density electrode group extremely difficult. The first problem occurs at the four corners of the rectangular case. When inserting the rectangular electrode group into the rectangular case, the active substance is easily peeled off of the electrode group at the four corners of the rectangular case. Then particularly, when the high density electrode group is being inserted into the battery case without any gaps, this drawback becomes much more important. Because the width of the electrode group and the inner width of the battery case are almost the same, the corners of the electrode group rub against the four corners of the battery case during the insertion process, and thus the active substance can be easily peeled off. In particular, when the electrode group is composed of many stacked layers of positive electrodes, negative electrodes and separators, it is fairly difficult to make its outer circumference with exactly the same shape as the inner circumference of the battery case. The outer shape of the electrode group is irregular due to components sliding out of position during the stacking process and to width differences between the positive electrode and the negative electrode. A further drawback is that when airtightly welding the cover to the battery case, a gas leak especially occurs at the corners where the active material has adhered and is impeding the tight closing of the battery case by the cover. To avoid this drawback, the width of the positive electrodes and of the negative electrodes is made narrower, and thus the substantial area of the pole plates becomes smaller and the volumetric energy density is reduced leading to another drawback.
Furthermore, rectangular batteries can for example be laminated with U-shape negative electrodes and positive electrodes nipped in between. The rectangular battery of this structure allows mass production with good efficiently because one electrode is laminated with a U-shape, and then with a reduced overall number of electrodes it is possible to augment the number of laminated plates. However, as shown in FIGS. 1 and 2, the drawback is that when inserting the electrode group of this structure into the battery case, the layer of active material on the surfaces near the U-shape core peels off due to rubbing against the open part of the battery case. As shown in FIG. 1, in the pole plate group 2 which is bent with a U-shape at the bottom of the pole plate, the active material 8 coated on the surface near the U-shaped part of the core 7 is peeled off by the open part when inserting the lower part into the open part of the battery case 1. Then, as shown in FIG. 2, in the electrodes bent in a U-shape at the lateral side of the electrode group 22, the peeling off of the active material 28 coated on the surface near the U-shape part of the core 27 occurs when inserting the pole plate group 22 into the battery case 21.
To prevent these drawbacks, which are particular to the rectangular batteries, a technique has been developed in which the surface of the electrode group 32 is covered with the metallic cover 312 which is bent with a horizontal U-shape and inserted into the battery case 31 (patent publication 6-4537). The rectangular battery of this structure has the characteristic of efficiently avoiding the falling off of the active material from the electrode group 32 when it is inserted into the battery case 31, because the metallic cover 312 can be inserted into the battery case 31 while rubbing against the inner face of the battery case 31. However, in the rectangular battery of this structure, because a metallic cover, which does not generate electricity, is placed between the electrode group 32 and the battery case 31, the size of electrode group 32 that can placed into the battery case 31, is reduced. For this reason, the metallic cover 312 has the drawback of reducing the volumetric energy density of the square type battery.
As above described, improving the volumetric energy density of the rectangular battery has been thoroughly researched, but the problem is that the realization of a solution is extremely difficult. Especially, the low price mass production of improved volumetric energy density rectangular batteries requires a furthermore difficult technique.
The first object of the present invention is to develop and realize the aforementioned technique and other important objects of the present invention are to prevent the adherence of the active material to the open part of the battery case when inserting the electrode group into the battery case and to propose a rectangular battery with an airtightly sealed battery case.
Then, other significant objects of the present invention are to improve the volumetric energy density of the rectangular battery with a fairly simple structure and furthermore to offer rectangular batteries which allow low price mass production.
Furthermore, the nickel hydrogen batteries, which recently are widely in use and which tend to replace the nickel-cadmium batteries, have the extreme characteristic of deteriorating the negative electrode due to the gaseous oxygen produced when the battery is fully charged. This is because the gaseous oxygen produced when the positive electrodes are fully charged, penetrates through the separators, spreads in the negative electrodes and adversely affects the negative electrodes.
FIG. 4 shows an electrode group of a nickel hydrogen battery, and the following is an explanation of the principle of the action of the gaseous oxygen which has a harmful effect on the negative electrodes.
The electrode group 42 is inserted into the rectangular sealed battery case. The electrode group is composed of positive electrodes 44 which use nickel hydroxide as the active material, and of negative electrodes 43 which use hydrogen absorbing alloy as the active substance 48. The positive electrodes 44 and the negative electrodes 43 are stacked with separators 45.
The nickel metal hydride battery is a battery that electrochemically uses the reversible reaction of emission of the occlusion of hydrogen in the case of the hydrogen absorbing alloy. FIG. 5 and FIG. 6 show the charge and discharge reactions of the negative electrodes 43 and of the positive electrodes 44. As shown in FIG. 5, when charging, by electrolysis reaction of water, the atomic hydrogen that has appeared on the surface of the hydrogen absorbing alloy 48A of the negative electrodes spreads and is included inside the inner part of the hydrogen absorbing alloy 48A. When discharging, as shown in FIG. 6, the occluded hydrogen reacts with the hydroxide ions at the surface of the hydrogen absorbing alloy and again becomes water. Therefore, the electrode reaction, which occurs via the hydrogen and the hydrogen absorbing alloy 48A, acts as a hydrogen tank.
Furthermore, when the charging process of the nickel metal hydride battery proceeds, and after the positive electrodes of small electrode capacity have been first fully charged, the gaseous oxygen appears by the following reaction. EQU 4OH.sup.- .fwdarw.2H.sub.2 O+O.sub.2 +4e.sup.-
Because the electrode capacity of the negative electrodes has been designed to be larger than the positive electrodes, gaseous hydrogen should theoretically not be produced. The gaseous oxygen that appears at the positive electrodes penetrates the separators and spreads out in the negative electrodes, oxidizing the hydrogen of the hydrogen absorbing alloy, which is in the charged condition, and water is formed according to the following reaction. EQU 4MH+O.sub.2 .fwdarw.4M+2H.sub.2 O
Then the water formed by this reaction is consumed by the charging reaction of the following formula: EQU M+H.sub.2 O+e.sup.- .fwdarw.MH+OH.sup.-
Furthermore, the gaseous oxygen, which was formed at the positive electrodes, is consumed by the following electrochemical reaction: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-
As above described, a sealed structure is realized in which the gaseous oxygen, which was formed at the positive electrodes, is consumed at the negative electrodes.
In a nickel metal hydride battery with an electrode group of this structure, the negative electrodes 43 are coated with the hydrogen absorbing alloy that is the active substance 48 on both faces of the core 47. In the nickel metal hydride battery of this structure, the gaseous oxygen, which is formed when the positive electrodes 44 are in the fully charged condition, deteriorates the negative electrodes 43. In particular, the negative electrodes 43, which do not face the positive electrodes 44, that is to say that the negative electrodes 43 in the region facing the battery case identified with the letter A added to the number in FIG. 4 deteriorate much more. Because the negative electrodes 43 in the region A do not face the positive electrodes 44, there is only a little occlusion and emission of hydrogen during the charging and discharging processes, and this is a region of poor electrochemical activity. Therefore, the negative electrodes 43 in the region A are regions of poor reactivity, but this is the region which is attacked by the oxygen that has penetrated into the separators 45 which leads to significant deterioration. When the negative electrodes 43 in this region deteriorate, the internal resistance of the battery augments, the cycle characteristics fall, and as a drawback, the high efficiency discharging quality is worsened.
Furthermore, the negative electrodes in the region A, which do not face the positive electrodes, are not really efficiently used during the charging and discharging processes because this is a region where the reactions of occlusion and emission of hydrogen are not efficient. The drawback is that inside the case, this is a region which is not efficiently used, and for the battery, this is an important loss of volumetric energy density. Especially, in the nickel metal hydride batteries, because a sealed structure is realized which absorbs the gaseous hydrogen in the negative electrodes and because the capacity of the negative electrodes is larger than the capacity of the positive electrodes, and further, if a region cannot be efficiently used, the volumetric energy density will be greatly reduced.
The second object of the present invention is to solve these drawbacks. The object of the present invention is to improve the volumetric energy density and to use the whole negative electrodes with more efficient charging and discharging processes, and furthermore, to offer nickel metal hydride batteries with improved cycle characteristics and a high efficiency discharging quality which allows effective prevention of battery deterioration.